Carbon Dioxide Gas Phase Reduction Apparatus and Method for Manufacturing a Porous Reducing Electrode-Supported Electrolyte Membrane

ABSTRACT

A carbon dioxide gas-phase reduction apparatus includes: an oxidation tank including an oxidation electrode; a reduction tank which is adjacent to the oxidation tank and into which carbon dioxide is supplied when the inside of the reduction tank is empty; and a porous reduction electrode-supporting electrolyte membrane disposed between the oxidation tank and the reduction tank. The porous reduction electrode-supporting electrolyte membrane is a joint body obtained by joining a porous reduction electrode formed by dispersing a first electrolyte membrane inside voids and a second electrolyte membrane. The second electrolyte membrane is disposed on the oxidation tank side. The porous reduction electrode is disposed on the reduction tank side, connected to the oxidation electrode by a conducting wire, and performs a reduction reaction with the carbon dioxide in the reduction tank by electrons flowing through the conducting wire.

TECHNICAL FIELD

The present invention relates to a carbon dioxide gas-phase reduction apparatus and a method for manufacturing a porous reduction electrode-supporting electrolyte membrane.

BACKGROUND ART

Conventionally, a technique for reducing carbon dioxide has attracted attention from a viewpoint of prevention of global warming and stable supply of energy. As a reduction apparatus that reduces carbon dioxide, there are a reduction apparatus using an artificial photosynthesis technique for reducing carbon dioxide by applying light energy such as sunlight, and an electrolytic decomposition apparatus that reduces carbon dioxide by applying electric energy from the outside (see Non Patent Literatures 1 to 4).

FIG. 2 of Non Patent Literature 1 illustrates a carbon dioxide gas-phase reduction apparatus using light irradiation. An electrolyte membrane is disposed between a left oxidation tank and a right reduction tank, and each of the oxidation tank and the reduction tank is filled with an aqueous solution. An oxidation electrode of gallium nitride (GaN) is put in the oxidation tank, a reduction electrode of copper (Cu) is put in the reduction tank, and the oxidation electrode and the reduction electrode are connected to each other by a conducting wire. Then, helium (He) is caused to flow into the aqueous solution in the oxidation tank, and carbon dioxide (CO₂) is caused to flow into the aqueous solution in the reduction tank.

At this time, when the oxidation electrode is irradiated with light, an electron/hole pair is generated and separated in the oxidation electrode, and oxygen (O₂) and protons (H⁺) are generated by an oxidation reaction of water (H₂O). Then, the protons move to the reduction tank through the electrolyte membrane, and electrons (e⁻) generated in the oxidation electrode move to the reduction electrode through the conducting wire. Thereafter, in the reduction electrode, hydrogen (H₂) is generated by bonding between the protons and the electrons, and a carbon dioxide reduction reaction is caused by the protons, the electrons, and carbon dioxide. By this carbon dioxide reduction reaction, carbon monoxide, formic acid, methane, and the like, which are utilized as energy resources, are generated.

CITATION LIST Non Patent Literature

-   Non Patent Literature 1: Satoshi Yotsuhashi, and 6 others “CO₂     Conversion with Light and Water by GaN Photoelectrode”, Japanese     Journal of Applied Physics 51, 2012 p. 02BP07-1-p. 02BP07-3 -   Non Patent Literature 2: Hiroshi Hashiba, and 4 others “Selectivity     Control of CO2 Reduction in an Inorganic Artificial Photosynthesis     System”, Applied Physics Express 6, 2013 p. 097102-1-p. 097102-4 -   Non Patent Literature 3: Yoshio Hori, and 2 others “Formation of     Hydrocarbons in the Electrochemical Reduction of Carbone Dioxide at     a Copper Electrode in Aqueous Solution”, Journal of the Chemical     Society 85(8), 1989 p. 2309-p. 2326 -   Non Patent Literature 4: Heng Zhong, and 2 others “Effect of KHCO3     Concentration on Electrochemical Reduction of CO2 on Copper     Electrode”, Journal of The Electrochemical Society 164(9), 2017 p.     F923-p. F927

SUMMARY OF INVENTION Technical Problem

In a case of a conventional carbon dioxide gas-phase reduction apparatus, carbon dioxide to be reduced is dissolved in an aqueous solution in a reduction tank, reaches a reduction electrode, and is reduced on a surface of the reduction electrode. However, since the aqueous solution is used as a means for mediating carbon dioxide, there is a limit to the concentration of carbon dioxide that can be dissolved in the aqueous solution, and since diffusion resistance of carbon dioxide in the aqueous solution is large, there is a limit to the amount of carbon dioxide that can be supplied to the reduction electrode.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a technique capable of improving efficiency of a carbon dioxide reduction reaction in a carbon dioxide gas-phase reduction apparatus.

Solution to Problem

A carbon dioxide gas-phase reduction apparatus according to an aspect of the present invention includes: an oxidation tank including an oxidation electrode; a reduction tank which is adjacent to the oxidation tank and into which carbon dioxide is supplied when the inside of the reduction tank is empty; and a porous reduction electrode-supporting electrolyte membrane disposed between the oxidation tank and the reduction tank, in which the porous reduction electrode-supporting electrolyte membrane is a joint body obtained by joining a porous reduction electrode formed by dispersing a first electrolyte membrane inside voids and a second electrolyte membrane, the second electrolyte membrane is disposed on the oxidation tank side, and the porous reduction electrode is disposed on the reduction tank side, connected to the oxidation electrode by a conducting wire, and performs a reduction reaction with the carbon dioxide in the reduction tank by electrons flowing through the conducting wire.

A method for manufacturing a porous reduction electrode-supporting electrolyte membrane according to an aspect of the present invention is a method for manufacturing a porous reduction electrode-supporting electrolyte membrane disposed between an oxidation tank including an oxidation electrode and a reduction tank into which carbon dioxide is supplied when the inside of the reduction tank is empty, the method including: impregnating a porous reduction electrode with an electrolyte dispersion in which a polymer material constituting an electrolyte membrane is dispersed; and superposing the porous reduction electrode impregnated with the electrolyte dispersion and an electrolyte membrane, and joining the porous reduction electrode and the electrolyte membrane by applying pressure while heating.

Advantageous Effects of Invention

The present invention can provide a technique capable of improving efficiency of a carbon dioxide reduction reaction in a carbon dioxide gas-phase reduction apparatus.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration example of a carbon dioxide gas-phase reduction apparatus according to Example 1.

FIG. 2 is a diagram illustrating a method for preparing a porous reduction electrode-supporting electrolyte membrane.

FIG. 3 is a diagram illustrating a state in which an electrolyte membrane is dispersed and formed in a porous reduction electrode.

FIG. 4 is a diagram illustrating a configuration example of a carbon dioxide gas-phase reduction apparatus according to Example 9.

FIG. 5 is a diagram illustrating a configuration example of a carbon dioxide gas-phase reduction apparatus according to Comparative Example 1.

FIG. 6 is a diagram illustrating a configuration example of a carbon dioxide gas-phase reduction apparatus according to Comparative Example 2.

FIG. 7 is a diagram illustrating a state in which an electrolyte membrane is dispersed and formed in an island shape in a porous reduction electrode.

DESCRIPTION OF EMBODIMENTS

Hereinafter, Examples of the present invention will be described with reference to the drawings. The present invention is not limited to Examples described later, and can be modified without departing from the gist of the present invention.

SUMMARY OF INVENTION

An object of the present invention is to provide a technique capable of improving efficiency of a carbon dioxide reduction reaction in a carbon dioxide gas-phase reduction apparatus. In order to achieve this object, the present invention has the following characteristics unlike a conventional carbon dioxide gas-phase reduction apparatus.

A first characteristic is that the inside of a reduction tank is filled with carbon dioxide in gas phase, and the carbon dioxide in gas phase is directly supplied to a reduction electrode. As a result, in the reduction tank, the concentration of carbon dioxide increases, and diffusion resistance of carbon dioxide decreases. As a result, the amount of carbon dioxide supplied to the reduction electrode increases, and efficiency of a carbon dioxide reduction reaction on the reduction electrode can be improved.

In this respect, in order to implement the carbon dioxide reduction reaction, a three-phase interface constituted by [electrolyte membrane-reduction electrode-carbon dioxide in gas phase] is required, but when the first characteristic is adopted, there is no aqueous solution in the reduction tank, and protons cannot move in a gas phase in the reduction tank. In addition, carbon dioxide in gas phase cannot move in the reduction electrode having no pore. As a result, the carbon dioxide reduction reaction cannot be implemented. Therefore, the present invention further has a second characteristic.

The second characteristic is that a porous reduction electrode having pores is used as the reduction electrode, and the porous reduction electrode and an electrolyte membrane are joined. As a result, a three-phase interface constituted by [electrolyte membrane-porous reduction electrode-carbon dioxide in gas phase] is formed, and therefore gas phase reduction of carbon dioxide on the porous reduction electrode can proceed even when the inside of the reduction tank is filled with carbon dioxide in gas phase.

However, only by joining the electrolyte membrane to the porous reduction electrode, the three-phase interface constituted by [electrolyte membrane-porous reduction electrode-carbon dioxide in gas phase] is limited only on a joining surface between the electrolyte membrane and the porous reduction electrode. Therefore, the present invention further has a third characteristic.

The third characteristic is that an electrolyte membrane is formed by being dispersed also inside voids of the porous reduction electrode. As a result, a reaction field of the carbon dioxide gas-phase reduction reaction increases, and therefore efficiency of the carbon dioxide reduction reaction on the porous reduction electrode can be improved.

That is, the present invention is characterized in that carbon dioxide in gas phase is directly supplied to a porous reduction electrode-supporting electrolyte membrane in which an electrolyte membrane is joined to a porous reduction electrode formed by dispersing an electrolyte membrane inside voids. This characteristic makes it possible to improve efficiency of a carbon dioxide reduction reaction on a reduction electrode.

Example 1 Configuration of Carbon Dioxide Gas-Phase Reduction Apparatus

FIG. 1 is a diagram illustrating a configuration example of a carbon dioxide gas-phase reduction apparatus 100 according to Example 1. The gas-phase reduction apparatus 100 is a reduction apparatus (artificial photosynthesis apparatus) that causes a carbon dioxide reduction reaction in a reduction electrode by irradiating an oxidation electrode with light. Hereinafter, the carbon dioxide gas-phase reduction apparatus 100 is simply referred to as the gas-phase reduction apparatus 100.

As illustrated in FIG. 1 , the gas-phase reduction apparatus 100 includes an oxidation tank 1 and a reduction tank 4 formed by dividing an internal space of one housing into two. The oxidation tank 1 is filled with an aqueous solution 3, and an oxidation electrode 2 made of a semiconductor or a metal complex is inserted into the aqueous solution 3. The reduction tank 4 adjacent to the oxidation tank 1 is filled with a gas of carbon dioxide or a gas containing carbon dioxide when the inside of the reduction tank 4 is empty.

The oxidation electrode 2 is, for example, a compound exhibiting photoactivity or redox activity, such as a nitride semiconductor, titanium oxide, amorphous silicon, a ruthenium complex, or a rhenium complex. The aqueous solution 3 is, for example, a potassium hydrogen carbonate aqueous solution, a sodium hydrogen carbonate aqueous solution, a potassium chloride aqueous solution, a sodium chloride aqueous solution, a sodium hydroxide aqueous solution, a potassium hydroxide aqueous solution, a rubidium hydroxide aqueous solution, or a cesium hydroxide aqueous solution.

Between the oxidation tank 1 and the reduction tank 4, a porous reduction electrode-supporting electrolyte membrane 20 in which a porous reduction electrode 5 formed by dispersing an electrolyte membrane (first electrolyte membrane) inside voids and an electrolyte membrane (second electrolyte membrane) 6 are joined is disposed. The electrolyte membrane 6 is disposed on the oxidation tank 1 side, and the porous reduction electrode 5 is disposed on the reduction tank 4 side. The oxidation electrode 2 and the porous reduction electrode 5 are connected to each other by a conducting wire 7.

A tube 8 is inserted into the oxidation tank 1 in order to cause helium to flow into the aqueous solution 3 in the oxidation tank 1. In the reduction tank 4, a gas input port 9 is formed at a bottom of the reduction tank 4 in order to cause carbon dioxide to flow into the reduction tank 4. Furthermore, a light source 10 is disposed so as to face the oxidation electrode 2 in order to operate the gas-phase reduction apparatus 100. The light source 10 is, for example, a xenon lamp, a pseudo sunlight source, a halogen lamp, a mercury lamp, sunlight, or a combination thereof.

Method for Preparing Porous Reduction Electrode-Supporting Electrolyte Membrane

A method for preparing the porous reduction electrode-supporting electrolyte membrane 20 will be described. The porous reduction electrode-supporting electrolyte membrane 20 is formed by joining the porous reduction electrode 5 and the electrolyte membrane 6.

The porous reduction electrode 5 is, for example, a porous body of copper, platinum, gold, silver, indium, palladium, gallium, nickel, tin, cadmium, or an alloy thereof, or a porous body of silver oxide, copper oxide, copper (II) oxide, nickel oxide, indium oxide, tin oxide, tungsten oxide, tungsten oxide (VI), or copper oxide. In addition, the porous reduction electrode 5 may be a porous metal complex having a metal ion and an anionic ligand.

The electrolyte membrane 6 is, for example, Nafion (registered trademark), Forblue, or Aquivion which is an electrolyte membrane having a carbon-fluorine skeleton. In addition, the electrolyte membrane 6 may be Selemion or Neosepta which is an electrolyte membrane having a hydrocarbon-based skeleton.

In Example 1, a copper porous body having a thickness of 1 mm and a porosity of 98% was used as the porous reduction electrode 5. Nafion which is a cation exchange membrane was used as the electrolyte membrane 6. As an electrolyte dispersion used at the time of preparation, a Nafion dispersion prepared by diluting Nafion dispersion (registered trademark) (Nafion content: 20 wt. %) manufactured by DuPont with pure water 200 times to adjust a Nafion content (=electrolyte content) to 0.05 wt. % was used. A solvent used for dilution is, for example, pure water, a lower alcohol, or a mixed liquid thereof. In Example 1, pure water was used.

First, the electrolyte membrane 6 is immersed in each of boiling nitric acid and boiling pure water in advance in order to improve proton mobility of the electrolyte membrane 6. Next, as step 1, the porous reduction electrode 5 is impregnated with an electrolyte dispersion (electrolyte content: 0.05 wt. %) in which a polymer material constituting an electrolyte membrane is dispersed. Thereafter, the porous reduction electrode 5 impregnated with the electrolyte dispersion is superposed on the electrolyte membrane 6 immersed in each of boiling nitric acid and boiling pure water, and the sample is disposed between two copper plates 30 a and 30 b as illustrated in FIG. 2 . Next, as step 2, this sample is disposed between hot plates 40 a and 40 b of a thermocompression bonding apparatus (hot press machine). Pressure is applied downward in a direction perpendicular to an upper surface of the porous reduction electrode 5 (to the electrode surface) while the sample is heated at a heating temperature of 150° C., and the sample is left for three minutes. Thereafter, the sample is quickly cooled and taken out. As a result, the porous reduction electrode-supporting electrolyte membrane 20 in which the porous reduction electrode 5 and the electrolyte membrane 6 are joined can be obtained. Note that the porous reduction electrode 5 after thermocompression bonding had a thickness of 0.2 mm and a porosity of 90%.

In step 1, since the porous reduction electrode 5 is impregnated with the electrolyte dispersion, the solution of the electrolyte dispersion 50 adheres to the surface and the inside of the porous reduction electrode 5 as illustrated in the enlarged diagram of FIG. 2 . When the hot press treatment in step 2 is performed in this state, a lower alcohol and pure water contained in the electrolyte dispersion 50 are vaporized by heating, and only Nafion contained in the electrolyte dispersion undergoes glass transition to form an electrolyte membrane (Nafion membrane) having a thickness of several hundred nm, whereby an electrolyte membrane (first electrolyte membrane) 60 is dispersed and formed on the surface and inside the porous reduction electrode 5 as illustrated in FIG. 3 . Since the porous reduction electrode 5 and the electrolyte membrane (second electrolyte membrane) 6 adhere to each other by the electrolyte membrane 60 dispersed and formed at an interface between the porous reduction electrode 5 and the electrolyte membrane 6, and a three-phase interface constituted by the dispersed and formed electrolyte membrane 60, the porous reduction electrode 5, and carbon dioxide in gas phase is formed inside the porous reduction electrode a reaction field of a carbon dioxide gas-phase reduction reaction increases, and the carbon dioxide reduction reaction efficiently proceeds at the three-phase interface.

Electrochemical Measurement and Measurement of Gas/Liquid Generation Amount

Electrochemical measurement and measurement of gas/liquid generation amount will be described.

The oxidation tank 1 is filled with the aqueous solution 3. As the oxidation electrode 2, a substrate obtained by epitaxially growing a thin film of gallium nitride (GaN) which is an n-type semiconductor and a thin film of aluminum gallium nitride (AlGaN) in this order on a sapphire substrate, and vacuum-depositing nickel (Ni) thereon and performing heat treatment to form a nickel oxide (NiO) co-catalyst thin film was used Then, the oxidation electrode 2 was disposed in the oxidation tank 1 so as to be immersed in the aqueous solution 3. As the aqueous solution 3, a 1.0 mol/L potassium hydroxide aqueous solution was used. As the light source 10, a 300 W high pressure xenon lamp (which cuts light having a wavelength of 450 nm or more, illuminance: 6.6 mW/cm²) was used, and the light source 10 was fixed such that a surface of the semiconductor photoelectrode of the oxidation electrode 2 on which the oxidation co-catalyst was formed (surface on which NiO was formed) was an irradiation surface. A light irradiation area of the oxidation electrode 2 was set to 2.5 cm².

Helium was caused to flow into the oxidation tank 1 from the tube 8, and carbon dioxide was caused to flow into the reduction tank 4 from the gas input port 9 at a flow rate of 5 ml/min and a pressure of 0.18 MPa. In this system, the carbon dioxide reduction reaction can proceed at the three-phase interface constituted by [electrolyte membrane-copper (porous reduction electrode)-carbon dioxide in gas phase] in the porous reduction electrode-supporting electrolyte membrane 20.

The oxidation tank 1 and the reduction tank 4 were sufficiently replaced with helium and carbon dioxide, respectively. Thereafter, the oxidation electrode 2 was uniformly irradiated with light using the light source 10. When the oxidation electrode 2 is irradiated with light, electrons flow between the oxidation electrode 2 and the porous reduction electrode 5. A current value between the oxidation electrode 2 and the porous reduction electrode 5 at the time of light irradiation was measured with an electrochemical measurement apparatus (Model 1287 Potentiogalvanostat manufactured by Solartron). In addition, gas and liquid in the oxidation tank 1 and the reduction tank 4 were collected at any time during light irradiation, and reaction products were analyzed with a gas chromatograph, a liquid chromatograph, and a gas chromatograph mass spectrometer. As a result, it was confirmed that oxygen was generated in the oxidation tank 1, and hydrogen, carbon monoxide, formic acid, methane, methanol, ethanol, and ethylene were generated in the reduction tank 4.

Example 2

In Example 2, the electrolyte content of the electrolyte dispersion in step 1 was set to 0.1 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 1.

Example 3

In Example 3, the electrolyte content of the electrolyte dispersion in step 1 was set to 0.5 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 1.

Example 4

In Example 4, the electrolyte content of the electrolyte dispersion in step 1 was set to 1.0 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 1.

Example 5

In Example 5, the electrolyte content of the electrolyte dispersion in step 1 was set to 5.0 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 1.

Example 6

In Example 6, a copper porous body having a thickness of 1 mm and a porosity of 90% was used in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The porous reduction electrode 5 after thermocompression bonding had a thickness of 0.2 mm and a porosity of 50%. The other conditions are all similar to those in Example 1.

Example 7

In Example 7, a copper porous body having a thickness of 1 mm and a porosity of 85% was used in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The porous reduction electrode 5 after thermocompression bonding had a thickness of 0.2 mm and a porosity of 25%. The other conditions are all similar to those in Example 1.

Example 8

In Example 8, a copper porous body having a thickness of 1 mm and a porosity of 81% was used in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The porous reduction electrode 5 after thermocompression bonding had a thickness of 0.2 mm and a porosity of 5%. The other conditions are all similar to those in Example 1.

Example 9 Configuration of Carbon Dioxide Gas-Phase Reduction Apparatus

FIG. 4 is a diagram illustrating a configuration of a carbon dioxide gas-phase reduction apparatus 100 according to Example 9. The carbon dioxide gas-phase reduction apparatus 100 is an apparatus for an electrolytic reduction reaction of carbon dioxide in gas phase (electrolytic reduction reaction apparatus). Hereinafter, the carbon dioxide gas-phase reduction apparatus 100 is simply referred to as the gas-phase reduction apparatus 100.

As illustrated in FIG. 4 , the gas-phase reduction apparatus 100 includes an oxidation tank 1 and a reduction tank 4 formed by dividing an internal space of one housing into two. The oxidation tank 1 is filled with an aqueous solution 3, and an oxidation electrode 2 made of a semiconductor or a metal complex is inserted into the aqueous solution 3. The reduction tank 4 adjacent to the oxidation tank 1 is filled with a gas of carbon dioxide or a gas containing carbon dioxide when the inside of the reduction tank 4 is empty. The oxidation electrode 2 is, for example, platinum, gold, silver, copper, indium, or nickel. Specific examples of the aqueous solution 3 are similar to those in Example 1.

Between the oxidation tank 1 and the reduction tank 4, a porous reduction electrode-supporting electrolyte membrane 20 in which a porous reduction electrode 5 formed by dispersing an electrolyte membrane (first electrolyte membrane) inside voids and an electrolyte membrane (second electrolyte membrane) 6 are joined is disposed. The electrolyte membrane 6 is disposed on the oxidation tank 1 side, and the porous reduction electrode 5 is disposed on the reduction tank 4 side. The oxidation electrode 2 and the porous reduction electrode 5 are connected to each other by a conducting wire 7. Specific examples of the porous reduction electrode 5 and the electrolyte membrane 6 are similar to those in Example 1.

A tube 8 is inserted into the oxidation tank 1 in order to cause helium to flow into the aqueous solution 3 in the oxidation tank 1. In the reduction tank 4, a gas input port 9 is formed at a bottom of the reduction tank 4 in order to cause carbon dioxide to flow into the reduction tank 4. Furthermore, a power supply 11 is connected to the conducting wire 7 in order to operate the gas-phase reduction apparatus 100.

Method for Preparing Porous Reduction Electrode-Supporting Electrolyte Membrane

The porous reduction electrode-supporting electrolyte membrane 20 is prepared by a procedure similar to that in Example 1.

Electrochemical Measurement and Measurement of Gas/Liquid Generation Amount

Electrochemical measurement and measurement of gas/liquid generation amount will be described.

The oxidation tank 1 is filled with the aqueous solution 3. As the oxidation electrode 2, platinum (manufactured by The Nilaco Corporation) was used. The oxidation electrode 2 was disposed in the oxidation tank 1 such that about 0.55 cm² of the surface area of the oxidation electrode 2 was immersed in the aqueous solution 3. As the aqueous solution 3, a 1.0 mol/L potassium hydroxide aqueous solution was used.

Helium was caused to flow into the oxidation tank 1 from the tube 8, and carbon dioxide was caused to flow into the reduction tank 4 from the gas input port 9 at a flow rate of 5 ml/min and a pressure of 0.18 MPa. In this system, the carbon dioxide reduction reaction can proceed at the three-phase interface constituted by [electrolyte membrane-copper (porous reduction electrode)-carbon dioxide in gas phase] in the porous reduction electrode-supporting electrolyte membrane 20. The area of the porous reduction electrode 5 to which carbon dioxide is directly supplied is about 6.25 cm².

The oxidation tank 1 and the reduction tank 4 were sufficiently replaced with helium and carbon dioxide, respectively. Thereafter, the oxidation electrode 2 and the porous reduction electrode 5 were connected to each other by the conducting wire 7 via the power supply 11, and a voltage of 2.5 V was applied to cause electrons to flow. A current value between the oxidation electrode 2 and the porous reduction electrode 5 when a voltage of 2.5 V was applied was measured with an electrochemical measurement apparatus. In addition, gas and liquid in the oxidation tank 1 and the reduction tank 4 were collected at any time during voltage application, and reaction products were analyzed with a gas chromatograph, a liquid chromatograph, and a gas chromatograph mass spectrometer. As a result, it was confirmed that oxygen was generated in the oxidation tank 1, and hydrogen, carbon monoxide, formic acid, methane, methanol, ethanol, and ethylene were generated in the reduction tank 4.

Example 10

In Example 10, the electrolyte content of the electrolyte dispersion in step 1 was set to 0.1 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 9.

Example 11

In Example 11, the electrolyte content of the electrolyte dispersion in step 1 was set to 0.5 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 9.

Example 12

In Example 12, the electrolyte content of the electrolyte dispersion in step 1 was set to 1.0 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 9.

Example 13

In Example 13, the electrolyte content of the electrolyte dispersion in step 1 was set to 5.0 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 9.

Example 14

In Example 14, a copper porous body having a thickness of 1 mm and a porosity of 90% was used in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The porous reduction electrode 5 after thermocompression bonding had a thickness of 0.2 mm and a porosity of 50%. The other conditions are all similar to those in Example 9.

Example 15

In Example 15, a copper porous body having a thickness of 1 mm and a porosity of 85% was used in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The porous reduction electrode 5 after thermocompression bonding had a thickness of 0.2 mm and a porosity of 25%. The other conditions are all similar to those in Example 9.

Example 16

In Example 16, a copper porous body having a thickness of 1 mm and a porosity of 81% was used in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The porous reduction electrode 5 after thermocompression bonding had a thickness of 0.2 mm and a porosity of 5%. The other conditions are all similar to those in Example 9.

Comparative Example 1

FIG. 5 is a diagram illustrating a configuration of a carbon dioxide gas-phase reduction apparatus according to Comparative Example 1 corresponding to Examples 1 to 8. The configuration of Comparative Example 1 is similar to that of a conventional carbon dioxide gas-phase reduction apparatus illustrated in FIG. 2 of Non Patent Literature 1.

The structure of the reduction tank 4 is different from that in FIG. 1 . The oxidation tank 1 and the reduction tank 4 are separated from each other only by the electrolyte membrane 6. A non-porous reduction electrode 5′ having no pore is inserted into the reduction tank 4. The inside of the reduction tank 4 is filled with an aqueous solution 12, and the non-porous reduction electrode 5′ is immersed therein. A tube 13 is inserted into the reduction tank 4 in order to cause carbon dioxide to flow into the aqueous solution 12.

As the aqueous solution 3 in the oxidation tank 1, a 1 mol/L sodium hydroxide aqueous solution was used. As the aqueous solution 12 in the reduction tank 4, a 0.5 mol/L potassium hydrogen carbonate aqueous solution was used. In addition, the non-porous reduction electrode 5′ was disposed so as to be immersed in the aqueous solution 12 using a copper plate (manufactured by The Nilaco Corporation) having an area of about 6 cm². The other constituent elements are similar to those in Example 1.

As a result of analyzing reaction products, it was confirmed that hydrogen, carbon monoxide, formic acid, methane, and ethylene were generated.

Comparative Example 2

FIG. 6 is a diagram illustrating a configuration of a carbon dioxide gas-phase reduction apparatus according to Comparative Example 2 corresponding to Examples 9 to 16.

The structure of the reduction tank 4 is different from that in FIG. 4 . The oxidation tank 1 and the reduction tank 4 are separated from each other only by the electrolyte membrane 6. A non-porous reduction electrode 5′ having no pore is inserted into the reduction tank 4. The inside of the reduction tank 4 is filled with an aqueous solution 12, and the non-porous reduction electrode 5′ is immersed therein. A tube 13 is inserted into the reduction tank 4 in order to cause carbon dioxide to flow into the aqueous solution 12.

As the aqueous solution 3 in the oxidation tank 1, a 1 mol/L sodium hydroxide aqueous solution was used. As the aqueous solution 12 in the reduction tank 4, a 0.5 mol/L potassium hydrogen carbonate aqueous solution was used. In addition, the non-porous reduction electrode 5′ was disposed so as to be immersed in the aqueous solution 12 using a copper plate (manufactured by The Nilaco Corporation) having an area of about 6 cm². The other constituent elements are similar to those in Example 9.

As a result of analyzing reaction products, it was confirmed that hydrogen, carbon monoxide, methane, ethylene, and formic acid were generated.

Experimental Results of Carbon Dioxide Reduction Reaction

Faraday efficiencies of the carbon dioxide reduction reaction in Example 1 to 16 and Comparative Examples 1 and 2 are presented in Table 1.

TABLE 1 Concentration of Porosity after Faraday efficiency electrolyte disper- thermocompres- of carbon dioxide sion in step 1 sion bonding reduction reaction (wt. %) (%) (%) Example 1 0.05 90 32 Example 2 0.1 90 34 Example 3 0.5 90 29 Example 4 1.0 90 5 Example 5 5.0 90 — Example 6 0.05 50 32 Example 7 0.05 25 21 Example 8 0.05 5 18 Example 9 0.05 90 30 Example 10 0.1 90 32 Example 11 0.5 90 27 Example 12 1.0 90 4 Example 13 5.0 90 — Example 14 0.05 50 29 Example 15 0.05 25 20 Example 16 0.05 5 17 Comparative — — 2 Example 1 Comparative — — 10 Example 2

As indicated in formula (1), the Faraday efficiency is a value indicating a ratio of a current value used in each reduction reaction to a current value flowing between electrodes at the time of light irradiation or voltage application.

Faraday efficiency of each reduction reaction=(current value of each reduction reaction)/(current value between oxidation electrode and reduction electrode)  (1)

The “current value of each reduction reaction” in formula (1) can be determined by converting a measured value of the generation amount of each reduction product into the number of electrons required for a generation reaction of each reduction product. The “current value of each reduction reaction” was calculated using formula (2), in which the concentration of a reduction reaction product is represented by A [ppm], a flow rate of a carrier gas is represented by B [L/sec], the number of electrons required for a reduction reaction is represented by Z [mol], the Faraday constant is represented by F [C/mol], and the molar volume of gas is represented by Vm [L/mol].

Current value[A] of each reduction reaction=(A×B×Z×F×10⁻⁶)/Vm  (2)

From Table 1, it can be understood that selectivity of carbon dioxide reduction in each of Example 1 to 3 is higher than that in each of Examples 4 and 5. It can be understood that selectivity of carbon dioxide reduction in each of Example 9 to 11 is higher than that in each of Examples 12 and 13.

In Examples 5 and 13, no product due to carbon dioxide reduction was detected, and therefore no measurement result of Faraday efficiency was obtained. This is considered to be because in Examples 4, 5, 12, and 13, the electrolyte membrane covered a metal surface of the porous reduction electrode 5, and as a result, supply of carbon dioxide to the porous reduction electrode 5 was significantly insufficient.

In addition, as illustrated in FIGS. 2 and 3 , when the porous reduction electrode 5 is impregnated with the electrolyte dispersion in step 1, the solution of the electrolyte dispersion 50 adheres to a surface of the porous reduction electrode 5, and when hot press treatment in step 2 is performed in this state, the electrolyte dispersion 50 is transferred to the electrolyte membrane 60 by heating, whereby the electrolyte membrane 60 is dispersed on the surface and inside the porous reduction electrode 5. However, when the concentration of the electrolyte dispersion 50 is 1.0 wt. % or more as in Examples 4, 5, 12, and 13, the surface of the porous reduction electrode 5 is completely covered with the electrolyte membrane 60, and therefore carbon dioxide cannot be supplied to the surface of the porous reduction electrode 5.

Meanwhile, when the concentration of the electrolyte dispersion 50 is 0.05 wt. % to 0.5 wt. % as in Examples 1 to 3 and 9 to 11, as illustrated in FIG. 7 , the electrolyte membrane 60 having a thickness of several μm is dispersed on the surface and at a void interface of the porous reduction electrode 5 and covers the porous reduction electrode 5 in an island shape. In this structure, a large number of three-phase interfaces each constituted by [reduction electrode-electrolyte membrane-carbon dioxide] are formed in the porous reduction electrode 5, and the carbon dioxide reduction reaction proceeds at the three-phase interfaces, and efficiency of the carbon dioxide reduction reaction is improved. Therefore, it is considered that the concentration of the electrolyte dispersion 50 used in step 1 is preferably less than 1.0 wt. %.

Furthermore, Faraday efficiency of carbon dioxide reduction in each of Examples 1 to 3, 6 to 8, 9 to 11, and 14 to 16 is largely improved as compared with that in each of Comparative Examples 1 and 2, and this indicates that the carbon dioxide reduction reaction selectively occurs. This is considered to be because in Examples 1 to 3, 6 to 8, 9 to 11, and 14 to 16, by directly supplying carbon dioxide in gas phase to the porous reduction electrode 5 without interposing an aqueous solution, the amount of carbon dioxide near the surface of the porous reduction electrode 5 increased, diffusion resistance of carbon dioxide decreased, the amount of carbon dioxide supplied to the porous reduction electrode 5 increased, and furthermore, the electrolyte membrane 60 was formed by being dispersed on the surface of the porous reduction electrode 5 to increase a reaction field.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, since carbon dioxide in gas phase is directly supplied to the porous reduction electrode-supporting electrolyte membrane 20 in which the electrolyte membrane 6 is joined to the porous reduction electrode 5, the concentration of carbon dioxide in the reduction tank 4 increases, and diffusion resistance of carbon dioxide near a surface of the porous reduction electrode 5 can be reduced. In addition, since the electrolyte membrane is formed by being dispersed inside the porous reduction electrode 5, a reaction field of a carbon dioxide gas-phase reduction reaction increases, and efficiency of a carbon dioxide reduction reaction in the porous reduction electrode 5 can be improved. Furthermore, in steps 1 and 2, since a void ratio of the porous reduction electrode 5 can be controlled in detail, and furthermore, the amount of the electrolyte dispersion to be dispersed inside the porous reduction electrode 5 can be defined, the reaction field can be easily controlled.

REFERENCE SIGNS LIST

-   -   1 Oxidation tank     -   2 Oxidation electrode     -   3 Aqueous solution     -   4 Reduction tank     -   5 Porous reduction electrode     -   5′ Non-porous reduction electrode     -   6 Electrolyte membrane     -   7 Conducting wire     -   8 Tube     -   9 Gas input port     -   10 Light source     -   11 Power supply     -   12 Aqueous solution     -   13 Tube     -   20 Porous reduction electrode-supporting electrolyte membrane     -   30 b Copper plate     -   40 b Hot plate     -   50 Electrolyte dispersion     -   60 Electrolyte membrane     -   100 Carbon dioxide gas-phase reduction apparatus 

1. A carbon dioxide gas-phase reduction apparatus comprising: an oxidation tank including an oxidation electrode; a reduction tank which is adjacent to the oxidation tank and into which carbon dioxide is supplied when an inside of the reduction tank is empty; and a porous reduction electrode-supporting electrolyte membrane disposed between the oxidation tank and the reduction tank, wherein the porous reduction electrode-supporting electrolyte membrane is a joint body obtained by joining a porous reduction electrode formed by dispersing a first electrolyte membrane inside voids and a second electrolyte membrane, the second electrolyte membrane is disposed on the oxidation tank side, and the porous reduction electrode is disposed on the reduction tank side, connected to the oxidation electrode by a conducting wire, and performs a reduction reaction with the carbon dioxide in the reduction tank by electrons flowing through the conducting wire.
 2. The carbon dioxide gas-phase reduction apparatus according to claim 1, wherein the first electrolyte membrane is formed at a void interface of the porous reduction electrode in an island shape.
 3. A method for manufacturing a porous reduction electrode-supporting electrolyte membrane disposed between an oxidation tank including an oxidation electrode and a reduction tank into which carbon dioxide is supplied when an inside of the reduction tank is empty, the method comprising: impregnating a porous reduction electrode with an electrolyte dispersion in which a polymer material constituting an electrolyte membrane is dispersed; and superposing the porous reduction electrode impregnated with the electrolyte dispersion and an electrolyte membrane, and joining the porous reduction electrode and the electrolyte membrane by applying pressure while heating.
 4. The method for manufacturing a porous reduction electrode-supporting electrolyte membrane according to claim 3, wherein the electrolyte dispersion has an electrolyte content of less than 1.0 wt. %. 